1. Field of the Invention
The present invention relates to antennas, and particularly to planar inverted-F antennas for wireless communication devices.
2. Related Art
Wireless communication devices, such as mobile phones, wireless cards, and access points, wirelessly radiate signals by use of electromagnetic waves. Thus, remote wireless communication devices can receive the signals without the need for cables.
In a wireless communication device, the antenna is a key element for radiating and receiving radio frequency signals. Characteristics of the antenna, such as radiation efficiency, orientation, frequency band, and impedance match, have a significant influence on performance of the wireless communication device. Nowadays, there are two kinds of antennas: built-in antennas and external antennas. In contrast to the external antenna, the size of the built-in antenna is smaller, and the body of the built-in antenna is protected and not easily damaged. Thus, the built-in antenna is commonly employed in wireless communication devices. Common built-in antennas include low temperature co-fired ceramic antennas and printed antennas. The low temperature co-fired ceramic (LTCC) antenna has good performance in high frequencies and at high temperatures, but is expensive. A common type of printed antenna is the planar inverted-F antenna. Compared to low temperature co-fired ceramic antennas, planar inverted-F antennas are small, light, thin, and inexpensive. Accordingly, planar inverted-F antennas are being used more and more in wireless communication devices.
In general, the planar inverted-F antenna is a printed circuit disposed on a substrate, and is used for radiating and receiving radio frequency signals. FIG. 3 is a schematic diagram of a typical planar inverted-F antenna. The planar inverted-F antenna includes a body 30, a feed wire 40, a shorting strip 50, and a metallic ground plane 20. The body 30 is used for radiating and receiving radio frequency signals, and includes a radiating end 31 and a shorting end 32. The shorting end 32 of the body 30 is connected to the metallic ground plane 20 by the shorting strip 50. The metallic ground plane 20 includes an opening 60. The feed wire 40 is electrically connected to the body 30, and is parallel to the shorting strip 50. The feed wire 40 is also electrically connected to a matching circuit (not shown) through the opening 60, for generating a matching impedance. The feed wire 40 is isolated from the metallic ground plane 20. The metallic ground plane 20, the body 30, the feed wire 40, and the shorting strip 50 are printed on the substrate 10.
In recent years, more attention has been paid to developing small-sized and low-profile wireless communication devices. Antennas, as key elements of wireless communication devices, have to be miniaturized accordingly. Although, the above-described planar inverted-F antenna is smaller than an external antenna, the profile of the above-described planar inverted-F antenna cannot be reduced efficiently, and so the profile of the corresponding wireless communication device cannot be reduced efficiently. Besides, requirements to the performance of the above-described planar inverted-F antenna have become higher and more rigorous. Therefore, what is needed is a planar inverted-F antenna with a compact profile and better performance.